1. Field of the Invention
The apparatus of this invention is designed to combine the liquid components of a mixture in accurate proportions. More specifically, a two-component adhesive system, including a resin and a solid hardener in water slurry is metered and proportioned.
2. Prior Art
In the production of laminated structural beams and other such products, it is necessary to produce an adhesive for face bonding the wood components together. The adhesive must be accurately made so that the finished products will meet critical load bearing specifications. A typical adhesive requires metering and mixing of a phenol-resorcinol resin with a liquid hardener slurry. Critical to resin performance is insuring the resin to hardener ratio is correct.
A major difficulty with the typical hardener is that it is abrasive and, in the water slurry form necessary for accurate flow control, causes such wear in conventional metering equipment that the critical ratios of hardener to resin are extremely difficult to maintain. The result can be laminated beams that fail design tests and sometimes even fail in use under load.
The prior art is also characterized by erratic control of adhesive from the metering-mixing apparatus. These control problems are caused by poor control strategies and inherently poor cleanup capability of the apparatus itself. Erratic spread rates and inability to start and stop adhesive flow as the workpiece leaves an extruder head are difficulties. Poor cleanup capability results in excess downtime for cleaning to maintain proper operation.
Both control and cleanup problems cause most adhesive system operators to use indirect application equipment. In these operations, the metering/mixing system discharges into a resin holding tank. A separate pump receives adhesive from the holding tank and dispenses it through an extruder head onto the workpiece. Not only does the indirect approach require capital, but adhesive losses are substantially increased as a result of added volumes that are circulated and the limited pot life of the adhesive.
A state-of-the-art liquid dispensing system is shown in FIG. 1. A hydraulic cylinder 1 dispenses a quantity of adhesive from a cylinder chamber 2 through a port 11 as the cylinder head 12 moves downwardly. The liquid component passes through a spool valve 15, specifically through ports 16, and proceeds, after mixing with other components, to application onto a workpiece. Simultaneously with discharge, the spool valve 15 admits through ports 17 liquid component, from a pressurized supply (not shown), into a hydraulic chamber 3 through a cylinder port 13. The cylinder chamber 3 fills as the cylinder head 12 proceeds downwardly. As soon as the desired amount of liquid is dispensed a spool rod 18 moves upwardly so that spool elements 19 occupy the position shown of dashed line spool element 19'. After the spool rod shifts, cylinder head 12 reverses direction, moving upwardly, now dispensing liquid component from chamber 3 through port 13. Chamber 2 receives a new supply of component on the upward stroke.
One commercial adhesive metering system includes two of the metering double acting cylinders 1 of FIG. 1, one each for delivery of resin and hardener. The cylinders are sized and interconnected through a centrally pivoted beam operated so that each rotation of the beam delivers an increment of resin and hardener properly proportioned for an adhesive application. Two spool valves 15 of the type shown in FIG. 1 control flows. However, each cylinder 1 and valve 15 has a number of sealing means 4 which are serious trouble spots in this arrangement. The liquid component can build up on cylinder and valve walls and work into the sealing means. This material, particularly if abrasive hardener, tends to rapidly wear seals, walls and operating spool or cylinder elements quickly, enabling component material to bypass the seals. The end result is what is really an inherent inability of conventional equipment to maintain critical ratio relationships.